I'm running a backup on Windows via cygwin to a clean local backup
directory. Since the backup is local, I thought it would be a
straight deal, but when I attempt the backup I get an unexpected
permissions error:

Ok. Please try using 1.1.17. The 1.1.x branch has two years worth of
Windows-specific bug fixes and corrections. If you still have problems
with that, send in a new report.

Secondly, do NOT edit your posts in that forum. That forum is
automatically sending each post (and each edit) as an email message to
the actual rdiff-backup users mailing list. Each one of your edits has
been spammed to the list, generating quite a few messages.

Frankly, it might be better if you simply subscribed to the mailing
list like a normal user: